A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate or by writing the pattern into the radiation-sensitive material using a particle beam, e.g. and electron-beam.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology substrate by measuring a part of the pattern and/or by measuring a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a narrow-band radiation beam of substantially monochromatic radiation and measure the intensity of the scattered radiation as a function of angle
Under a certain lithographic setting (e.g., lithographic apparatus, wavelength, NA, and so on), the amount of defocus and exposure dose variations that a circuit design can tolerate, while still producing functional chips, is called the design's “process window.” The process window is often characterized as an area or region in the two-dimensional Focus Exposure Matrix plot (further also referred to as FEM plot), where “F” represents the focus value or defocus, and “E” represents exposure dose variation. Also other settings of the lithographic apparatus or lithographic tool (e.g., NA) and other processing parameters may have an impact on the process window.
When a layer on a substrate is exposed to a particular Focus and Exposure condition within the process window, that layer on the substrate will most likely be functional (again, apart from local non-systematic defects that may have occurred, such as dust particles locally damaging functionality). When all layers on the substrate are exposed at Focus and Exposure conditions within their respective process windows, the complete dice on the substrate will be functional (apart from non-systematic defects as mentioned before). When a layer on the substrate is exposed to a further particular Focus and Exposure condition outside the process window, that layer and probably all dice on the substrate will not be functional. Incorrect determination of the process window may result a Focus and Exposure condition being used which falls within the incorrectly determined process window (so the layer is expected to be functional), but results in defective dice.